As is well known, when shifting from a straight-ahead driving state of to a cornering state on four-wheel-drive (4WD) vehicles, a so-called tight-corner braking phenomenon takes place, because of insufficient absorption of the wheel-speed difference between front and rear road wheels. As briefly described hereunder, Japanese Patent Provisional Publication No. 2001-253261 (hereinafter is referred to as “JP2001-253261”) teaches the avoidance of a tight-corner braking phenomenon peculiar to four-wheel-drive vehicles. In the power distribution controller described in JP2001-253261, a turning radius R of the 4WD vehicle is estimated based on the vehicle speed and lateral acceleration exerted on the 4WD vehicle during cornering. When the estimated turning radius R is less than a predetermined threshold value R0, a distribution ratio of a secondary-drive-wheel torque to a primary-drive-wheel torque is decreasingly compensated for based on the estimated turning radius, so as to optimize the secondary-drive-wheel-to-primary-drive-wheel torque distribution ratio and thus to properly avoid the tight-corner braking phenomenon.